This invention relates to a manufacturing method for a silicon single crystal, using a CZ method (a Czochralski Method) pulling the silicon single crystal from a silicon metal contained in a crucible. More particularly, the invention relates to a method obtaining a high quality silicon single crystal by controlling an interstitial oxygen concentration in the silicon single crystal with a high precision and a silicon single crystal that can be realized by the method for the first time.
In order to produce a silicon single crystal wafer used in fabrication of semiconductor devices, there has been widely used a silicon single crystal manufactured by a Czochralski method (hereinafter also referred to as a CZ method or a pulling method), which is advantageous in growing a large size crystal. A manufacturing method according to the Czochralski method is, as is well known to the general public, such that a seed crystal is dipped into a silicon melt obtained by melting in a quartz crucible of a single crystal growth apparatus and thereafter, the dipped seed crystal in the melt is slowly pulled upward while rotating the seed crystal in a direction opposite to that of the quartz crucible, resulting in growth of a silicon single crystal in an almost cylindrical shape.
Since a quartz crucible is used for holding a silicon melt in a manufacturing method for a silicon single crystal according to a Czochralski method, the quartz of a crucible wall reacts with the silicon melt to dissolve oxygen atoms into the silicon melt and ad as a result, the silicon is incorporated into the single crystal during growth through the melt. For this reason, a silicon single crystal manufactured by a CZ method includes oxygen atoms in supersaturation and a mechanical strength of the silicon single crystal in processing into wafers increases in the presence of the oxygen atoms in supersaturation. Therefore, characteristically, not only does a silicon wafer from such a single crystal come to have a large resistance force against various thermal strains received when the semiconductor elements are fabricated thereon, but dislocations such as slippage are also difficult to be produced therein (see K. Sumino: Semiconductor Silicon 1981, Electrochem. Soc., Penington 1981 p. 208).
Furthermore, while oxygen in super saturation existing in a wafer is transformed into oxide precipitates (bulk micro defects, which is hereinafter abbreviated as BMD) by a heat treatment of the wafer after a single crystal is processed into wafers, but in a case where BMDs are introduced in a region remote from an element forming region in a surface layer of the silicon wafer, the BMDs serves as getter sinks collecting impurities existing in the wafer and therefore, capture various impurities introduced into a wafer during an element""s forming process, thus performing a role to keep clean the element forming region. This method is called intrinsic gettering (hereinafter abbreviated as IG) and IG has been widely used as a gettering method when elements are fabricated on a wafer.
However, on the other hand, as interstitial oxygen existing in a wafer increases, a forming amount of BMDs also increase and with the forming amount in excess, especially, in a wafer surface layer which is an element forming region, the BMDs are a cause for a leakage failure at element junction interfaces, which in turn invites deterioration in element characteristics, resulting in loss of an essential function of a semiconductor integrated circuit by chance. Therefore, it is very important when semiconductor elements are fabricated in the surface layer of a wafer to keep an interstitial oxygen concentration in a CZ method silicon single crystal at a proper value, which produces a problem of how to control an interstitial oxygen concentration in a crystal at proper value in order to take care of a high density and high degree of integration of a semiconductor device, such that the control of an interstitial oxygen concentration have further become an important factor in maintaining a product quality of an integrated circuit.
In growth of a silicon single crystal by a CZ method, as an amount of a silicon melt contained in a quarts crucible decreases, a contact area between a wall of the quartz crucible and the silicon melt decreases. As a result, oxygen dissolved from the crucible wall also decreases, in turn an oxygen concentration in the latter half of a grown crystal decreases to a value equal to or less than a desired value, and thereby, there arises a chance to be unable to ensure a necessary quality. As a measure to improve this problem, proposal has been made on a method for adjusting an amount of oxygen supplied into the silicon melt from the quartz crucible wall by increasing a rotation speed of a crucible with progress in growth of a single crystal, that is as a pulled amount of a single crystal ingot is increased.
In this method, however, it has been said to be hard to achieve a high precision control of an interstitial oxygen concentration adapting to growth of a single crystal since, in a quartz crucible of as large a size as a diameter in excess of 200 mm, there exists a distance between a crucible wall supplying oxygen and a crystal growth interface, which in turn causes a delay in time for a melt to reach the crystal growth interface. As a result, in a case where a large size crystal equal to or more than, especially, 200 mm in diameter is pulled, there has been an inevitable limitation on control to suppress variations in interstitial oxygen concentration due to the above inconvenience.
On the other hand, as a method for controlling an interstitial oxygen concentration in a silicon single crystal, there is disclosed in JP A 81-104791, for example, a MCZ method whereby to grow a single crystal under application of a magnetic field (a magnetic field applied Czochralski method, which is also called magnetic field applied pulling method), in which a proposal is made on a technique that an interstitial oxygen concentration is efficiently reduced. In one aspect of this method, however, somewhat troublesome faults have been encountered in setting of and adjustment for growing conditions of a single crystal since an oxygen concentration is excessively low by chance depending on growth conditions for a single crystal when a BMD density is desired at a necessary and sufficient level in a wafer, therefore, a special technique is required to be performed for achieving a high oxygen concentration on a specific target quality of a crystal and a growth state of the crystal.
As a means for solving such problems, methods have been proposed: one of JP A 92-31386 in which a magnetic field is altered in strength according to a growth length of a crystal such that an interstitial oxygen concentration along the crystal growth axis direction is kept constant and the other of JP A 93-194077 in which a rotation speed of a crucible filled with a melt is controlled so as to adjust an oxygen concentration in a grown crystal.
In growth of a silicon single crystal according to a MCZ method, interstitial oxygen existing in a silicon single crystal originates from oxygen in the silicon melt dissolved from a quartz crucible, similar to an ordinary CZ method, and the oxygen in the melt is incorporated into the crystal through a crystal growth interface. In the MCZ method, however, by growing a silicon single crystal under application of a magnetic field to a silicon melt, a turbulent in the melt produced by thermal convection in the melt and rotation of a crucible can be efficiently suppressed, so an amount of oxygen supplied to a region in the vicinity of a crystal growth interface is caused low, with the result that a single crystal of a low oxygen concentration can be grown. This is a mechanism enabling a low oxygen crystal to be grown in an MCZ method.
According to a prior art MCZ method, however, in use of a large size quarts crucible for growth of a single crystal as large a diameter as to exceed 200 mm, as described above, convection in a silicon melt is extremely suppressed by a magnetic field and thereby, more of a delay in time arises for a melt portion on a crucible wall surface to reach a growth interface, which in turn results in more of difficulty in terms of high precision control of an interstitial oxygen concentration adapting to growth of a crystal. In fact, a method is disclosed in JP A 97-235192, in which rotation speeds of a crucible and a single crystal are adjusted in respective predetermined ranges in order to suppress variations in an interstitial oxygen concentration along the growth axial direction when a silicon single crystal is pulled by a MCZ method. According to the disclosed technique, even in a small diameter silicon single crystal of the order 6 inches (about 150 mm) in diameter, a variation in the concentration still remains at an extent of within xc2x10.05xc3x971018 atoms/cm3 (xc2x10.5xc3x971017 atoms/cm3), from which expectation is deduced that more of a variation will be produced for a larger diameter silicon single crystal. Note that, in a MCZ method, although a proposal is made on a method adjusting an oxygen amount supplied into a silicon melt from a crucible wall in which as an pulling amount of a single crystal ingot increases, a magnetic field applied to the silicon melt is reduced in strength, this has not been able to be a decisive solution as great as to be dramatic improvement of suppression of variations in interstitial oxygen concentration in pulling a single crystal of equal to or more than 200 mm in diameter either.
The invention has been made in light of the above problems and it is accordingly an object of the present invention to provide a manufacturing method for a high quality silicon single crystal whose interstitial oxygen concentration along a crystal growth axis direction is controlled with a high degree of precision and provide a silicon single crystal obtained by the same method.
In order to solve the above problem, the invention is directed to a method for manufacturing a silicon single crystal from a silicon melt contained in a crucible in a silicon single crystal growth furnace by a CZ method in which not only is a MCZ method (a magnetic field applied pulling method) performing single crystal pulling under application of a magnetic field adopted, but also a flow rate of an inert gas flowing in the growth furnace during growth of the silicon single crystal and/or a pressure in the growth furnace is altered according to a pulling amount of the silicon single crystal to adjust an interstitial oxygen concentration therein.
In the invention, in order that an oxygen concentration in a silicon single crystal obtained by a CZ method (a Czochralski method) is restricted to a predetermined level or less during pulling of the silicon single crystal, and that a concentration of oxygen incorporated into the silicon single crystal is stabilized, the growth of a silicon single crystal is performed while suppressing convection in a melt with adoption of an MCZ method. As a method unique to the invention, a flow rate of an inert gas flowing in the growth furnace during growth of the silicon single crystal and/or a pressure in the growth furnace is altered according to a pulling amount of the silicon single crystal, thereby, enabling a suppressed level of variations in interstitial oxygen concentration, which was regarded as impossible in a CZ method in the prior art, to be realized.
Furthermore, a silicon single crystal of the invention can be realized for the first time by adoption of the manufacturing method of the invention, which is a silicon single crystal of 200 mm or more in diameter grown using a CZ method pulling a single crystal from a silicon melt having a variation in interstitial oxygen concentration at a center of a single crystal along the crystal growth axis direction confined within xc2x10.2xc3x971017 atoms/cm3 of the average thereof contrary to the average oxygen concentration at a center of the crystal growth axis. Note that in this specification, an interstitial oxygen concentration is a measurement obtained according to a method stipulated in F-121 of ASTM (1979). Especially, in a large diameter single crystal of 200 mm or more (for example, 8 inches), 250 mm or more (for example, 10 inches) or 300 mm or more (for example, 12 inches), it was traditionally very difficult to restrict variations in interstitial oxygen concentration to a sufficiently low level even by adopting an MCZ method. By adoption of a manufacturing method of the invention, however, it can be realized to very effectively suppress variations in interstitial oxygen concentration of a single crystal in such a large diameter, thereby enabling a single crystal having a variation within the above numerical range to be actually obtained. Note that it is practical in growth of a silicon single crystal that an average (that is the central value of variations) of values of an interstitial oxygen concentration in all of the silicon single crystal is controlled on the order in the range of from 6xc3x971017/cm3 to 12.5xc3x971017/cm3, which are practical variations for the range of low oxygen concentration in growth of a silicon single crystal. According to a manufacturing method of the invention, by altering a flow rate of an inert gas flowing in the growth furnace during growth of a silicon single crystal and/or a pressure in the growth furnace according to a pulling amount of the silicon single crystal, an oxygen concentration in the melt can be efficiently adjusted with less of a delay in time compared with a method using a change in magnetic field or a method for controlling an oxygen concentration by adjusting a rotation speed of a crucible; therefore a concentration of oxygen incorporated into the crystal can be stabilized. According to the above manufacturing method, by altering a flow rate of an inert gas flowing in the growth furnace during growth of the silicon single crystal or a pressure in the growth furnace, an amount of oxygen evaporating as an oxide from a surface of the melt in the vicinity of a crystal growth interface can be easily adjusted, and thereby, an oxygen amount included in the silicon melt can be controlled with ease. Furthermore, by adopting such a method, an oxygen concentration in the surface layer of the silicon melt can be controlled with more of ease, thereby, enabling control of an oxygen amount in the melt existing in the vicinity of a crystal growth interface with a good precision.
In the manufacturing method for a silicon single crystal of the invention, a flow rate of an inert gas flowing in the growth furnace can be reduced as a pulling amount of the silicon single crystal increases. With progress in growth of the crystal, a silicon melt in a crucible decreases and in turn a contact area between a crucible wall and the silicon melt decreases, which causes an oxygen amount of supply from the crucible to be reduced. Therefore, by decreasing a flow rate of an inert gas flowing in the growth furnace, for example an inert gas flowing down toward the melt surface from above in the growth furnace, an amount of SiO (silicon monoxide) evaporated off away from the melt surface by the inert gas decreases, thereby enabling prevention of reduction of an oxygen concentration in the surface layer of the melt to a value equal to or less than a desired value. With such a technique adopted, an oxygen concentration in the melt in the vicinity of a crystal growth interface is kept at a proper value and alteration in oxygen concentration in the crystal occurring in company with decrease in the melt can be effectively suppressed. The invention can be effectively adopted in a case where a large diameter crystal of 200 mm or more in diameter is grown by use of an especially large size quartz crucible (for example, with a diameter of 500 mm or more).
In this case, a flow rate of an inert gas flowing in the growth furnace during growth of a silicon single crystal is desirably adjusted in the range of from 40 to 300 l/min. With no relation to an amount of a melt in a crucible, an effect of removing an oxide evaporating from the surface of the melt becomes low with a flow rate of the inert gas flowing in the furnace equal to or less than 40 l/min and thereby, a case arise by chance where it is hard to properly control an oxygen concentration in the melt. Furthermore, an effect of removing an oxide evaporating off away from the melt becomes low; therefore, not only does the upper end of a quarts crucible and an oxide attach onto portions which have low temperature in the growth furnace to contaminate the inner structure of the growth furnace, but also the attachment has a chance to fall onto the surface of the melt to disturb cystallization of silicon. On the other hand, the inert gas flow rate more than necessary is also problematic. When an amount of the inert gas is in excess, an amount of oxygen to be removed from the melt surface increases and an amount of oxygen supplied from the crucible increases in a corresponding manner, whereby deterioration of a quartz crucible is accelerated, resulting in difficulty in operation for a long time. Furthermore, when an amount of the inert gas flowing over the melt surface increases, ripples arise on the melt surface, by which there arises a fear that the ripples produce an inconvenience such as to create dislocations in a single crystal in growth and further, which brings a disadvantage in an aspect of manufacture cost because of increase in consumption of the inert gas. From this viewpoint, a flow rate flowing in the furnace is desirably in the range of 300 l/min or less and more desirably in the range of from 60 to 200 l/min. Note that a flow rate of an inert gas flowing in the furnace has a chance that a more desirable numerical range thereof differs according to a size of a growth furnace, a size of a crucible and a diameter of a silicon single crystal to be grown. To be concrete, the upper limit of a gas flow rate during crystal growth in a growth furnace has a character to shift upward with increase in volume thereof and, for example, a flow rate of an inert gas can have, in a good probability, a case to exceed 300 l/min in a large size growth furnace by which a single crystal having a diameter exceeding 300 mm is grown. However, with a growth furnace having any larger size, when a silicon single crystal is grown using an MCZ method therein, by controlling a flow rate of an inert gas flowing in the growth furnace so as to match with a pulling amount of the single crystal, an interstitial oxygen concentration in the single crystal can be controlled properly, which circumstances do not change by a size of the growth furnace.
An inert gas flowing in the growth furnace is preferably argon gas. When the inert gas is argon gas, not only can stable crystal growth be ensured without performing an unnecessary chemical reaction in the growth furnace, but also an influence on a crystal quality is small, thereby enabling a high quality crystal to be obtained.
Furthermore, in a manufacturing method for a silicon single crystal of the invention, a pressure in a crystal growth furnace can also be increased with increase in a pulling amount of the silicon single crystal. As a pulling amount of the silicon single crystal increases, an amount of a melt decreases, thereby, reducing an oxygen concentration in the melt. By increasing a pressure in the growth furnace, however, an amount of SiO evaporating from the surface of the melt can be suppressed. Therefore, when a pressure in the growth furnace is raised so as to match with growth of the silicon single crystal, an evaporation amount of SiO decreases to stabilize an oxygen amount in the silicon melt. A method for controlling an oxygen concentration by adjusting a pressure in a growth furnace has an effect of suppressing consumption of an inert gas because of no relation with an amount of the inert gas flowing in the furnace and is also excellent in an aspect of manufacturing cost of a single crystal. A pressure in a growth furnace during growth of a silicon single crystal is preferably adjusted in the range of from 40 to 300 mbar. Under a pressure in the growth furnace of 40 mbar or less, an amount of an oxide such as SiO evaporating off away from the surface of a melt increases to a level in excess of a necessary value and an amount of supply of oxygen from a quartz crucible wall increases to thereby deteriorate a durability of the crucible; therefore, there arise a case where single crystal growth over a long time becomes difficult. On the other hand, in a case where operation is performed under a condition in excess of 300 mbar, while an amount of oxide evaporating from the melt can be suppressed, a pressure in the growth furnace becomes excessively high, whereby SiO attaches on a furnace wall, a hot zone (a structure in the furnace) and others to disable visual observation in the furnace or crystal growth suffers falling-off of the attached SiO onto the melt resulting in disturbance of growth of a crystal by chance, which is not preferable from the viewpoint of actual operation. A pressure in the growth furnace is desirably adjusted in the range of from 60 to 200 mbar.
Then, since, by using a method of the invention, an oxygen concentration included in a melt in the vicinity of a growth interface can be directly controlled, therefore, control of an oxygen concentration with no delay in time can be realized, thereby enabling control of oxygen incorporated into the crystal with good precision. Especially in growth of a large size single crystal of 200 mm or more (for example, 8 inches or more) in diameter, a high strength magnetic field is applied to perform an operation; therefore, the effect of the present invention is exerted conspicuously. To be concrete, a case can be exemplified in which an operation is performed under a condition that a magnetic field applied to a silicon melt in a growth furnace growing a silicon single crystal therein has a strength of 3000 G or higher at a position where the magnetic field reaches the maximum strength thereof, wherein the strength 3000 G is a magnetic field strength expressed in terms of a magnetic flux density with a gauss as a unit. Note that while there is no special value of the upper limit as a value of the magnetic field, the upper limit is preferably selected on the order of 10000 G as a practical range in consideration of a size, a running cost of a magnetic field generation apparatus and others.
As a magnetic field applied to a silicon melt in a growth furnace growing a silicon single crystal, for example, a horizontal magnetic field can be adopted. In this case, growth of a large size single crystal as described above is desirably performed at a central magnetic field strength of the horizontal magnetic field of 3000 G or more, wherein the central magnetic field strength corresponds to the maximum magnetic field strength. In a CZ method performing crystal growth under application of a horizontal magnetic field, an effect of suppressing convection throughout all of the bulk of a melt in a quarts crucible is exerted under an influence of the horizontal magnetic field. Especially, since the center of the magnetic field is located on a crystal growth axis in the melt, a suppressing effect on convection in the vicinity of a crystal growth interface is large and it takes a time from when a rotation speed of a crucible is altered till an influence of the change in rotation speed is exerted on an oxygen concentration in the melt in the vicinity of a crystal growth interface, having resulted in a problem in regard to precise control of an amount of oxygen incorporated into a crystal. By adopting of a manufacturing method of the invention, however, such a problem can be solved effectively.
In a manufacturing method of the invention, a flow rate of an inert gas and/or a pressure in a growth furnace to be altered so as to match with a pulling amount of a single crystal can be calculated based on measurements of an interstitial oxygen concentration in silicon single crystals having been pulled in a previous time. In growth of a silicon single crystal using a manufacturing method of the invention, by performing setting and adjusting current growth conditions based on oxygen concentration data on silicon single crystals pulled in a previous time and operating condition data such as a flow rate of an inert gas, a pressure in a growth furnace and others in the previous time of the growth, a single crystal with a more stable quality can be obtained. That is, if a portion in a low oxygen concentration exists in a part of a grown single crystal; in the next run of crystal growth, growth conditions can be adjusted such that a flow rate of an inert gas is reduced at a timing just when the part in a low oxygen concentration is in the growth or a pressure in the furnace is increased at the same timing. Contrary to this, in a part where an oxygen concentration is higher than a target value, if an adjustment of operating conditions is performed such that a gas flow rate increases or a pressure in the furnace decreases, thereby, a single crystal can be easily obtained with a stable oxygen concentration distribution along a crystal axis direction with ease. Note that the gas flow rate and the pressure in the furnace can be adjusted simultaneously.